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We present simultaneous optical and near-infrared spectra obtained during the
2021 outburst of the black hole transient 4U 1543-47. The X-ray
hardness-intensity diagram and the comparison with similar systems reveal a
luminous outburst, probably reaching the Eddington luminosity, as well as a
long-lasting excursion to the so-called ultra-luminous state. VLT/X-shooter
spectra were taken in two epochs 14 days apart during the early and brightest
part of the outburst, while the source was in this ultra-luminous accretion
state. The data show strong H and HeI emission lines, as well as
high-excitation HeII and OIII transitions. Most lines are single-peaked in both
spectra, except for the OIII lines that exhibit evident double-peaked profiles
during the second epoch. The Balmer lines are embedded in broad absorption
wings that we believe are mainly produced by the contribution of the A2V donor
to the optical flux, which we estimate to be in the range of 11 to 14 per cent
in the $r$ band during our observations. Although no conspicuous outflow
features are found, we observe some wind-related line profiles, particularly in
the near-infrared. Such lines include broad emission line wings and skewed red
profiles, suggesting the presence of a cold (i.e. low ionisation) outflow with
similar observational properties to those found in other low-inclination black
hole transients.
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